Use uptimeMillis instead of currentTimeMillis for interval calculation.

uptime is guaranteed monotonic; currentTime may go backwards in the event
of NTP updates or other wackiness.

Test: manual sanity check of logs; android.support.test.metricshelper.MetricsAssertsTest passes
Bug: 37538546, 18584861

Change-Id: I222b20be9d561e1184b11155fe44c574d90872a5